1. Field of the Invention
The invention relates to sunscreen compositions, particularly those in lotion and cream form.
2. The Related Art
Sunscreen compositions are commonly used during outdoor work or leisure for protection of exposed skin against painful sunburn. Many effective sunscreen preparations are sold commercially or are described in cosmetic or pharmaceutical literature. In general, sunscreen preparations are formulated in the form of creams, lotions or oils containing as the active agent an ultraviolet radiation absorbing chemical compound. The active agent functions by blocking passage of erythematogenic radiation thereby preventing its penetration into the skin.
The ideal sunscreen formulation should be non-toxic and non-irritating to skin tissue and be capable of convenient application in a uniform continuous film. The product should be sufficiently chemically and physically stable so as to provide an acceptable shelf life upon storage. It is particularly desirable that the preparation should retain its protective effect over a prolonged period after application. Thus, the active agent when present on the skin must be resistant to chemical or photodegradation, to absorption through the skin, and to removal by perspiration, skin oil, or water. For aesthetic reasons, the product should be substantially odorless (or be capable of being scented) and be non-staining to the skin or clothing.
Sunscreen agents in the order of decreasing effectiveness may be categorized as either highly chromophoric monomeric organic compounds, inorganic compounds and minimally chromophoric polymeric organic solids.
U.S. Pat. No. 5,219,558 (Woodin, Jr. et al.) and U.S. Pat. No. 4,919,934 (Decker et al.) disclose photoprotection compositions wherein the active sunscreen agents are of the chromophoric monomeric organic compound variety. The examples feature the commercially common sunscreens such as octyl methoxycinnamate (Parsol MCX), benzophenone-3 (Oxybenzone) and octyl dimethyl PABA.
Chromophoric monomeric organic compounds are subject to certain problems. These compounds when present on the skin must be resistant to removal by perspiration, skin oils or water. Formulations containing these materials therefore require additives to ensure substantivity. Yet, even with the best additives waterproofing and rub off resistance is never fully accomplished. Another and perhaps more important problem is that of skin irritation. See U.S. Pat. No. 5,041,281 and U.S. Pat. No. 4,917,883 both to Strobridge reporting oil-in-water emulsion sunscreens waterproofed with, for instance, copolymers of ethylene and vinyl acetate. Some people are quite sensitive to organic molecules with chromophoric groups. Adverse allergic reactions can result. Therefore, it would be quite desirable to minimize the level of such compounds in any sunscreen compositions. Total replacement of chromophoric organic compounds, while desirable, is presently not feasible for high SPF compositions that also require certain types of aesthetics.
Inorganic particulate compounds such as titanium dioxide have been employed as sunscreen agents. In fact, titanium dioxide is quite popular with marketers advertising them as "natural sunscreens". The problem with inorganic particulate compounds is that high SPF values can only be achieved with high concentrations of these materials. Unfortunately, aesthetics suffer at such high concentrations. Clear formulas become opaque. High loadings also tend to form visible white films on the skin which consumers perceive negatively.
Polymeric organic particulates are a final category of materials which have found use in sunscreen formulations. U.S. Pat. No. 5,008,100 (Zecchino et al.) reports oil-in-water emulsions containing polyethylene particles as a co-active sunscreen agent along with the traditional chromophoric organic compounds. Similar to the inorganic materials, polymeric particles are limited in their sunscreen effectiveness. High amounts of such materials will have adverse effects upon the formula aesthetics.
Accordingly, it is an object of the present invention to provide a sunscreen composition that maximizes the sun protection factor but minimizes the level of chromophoric monomeric organic compound.
Another object of the present invention is to provide a sunscreen composition in the form of an oil and water emulsion that exhibits improved aesthetics when applied to the skin.
Yet another object of the present invention is to provide a sunscreen composition having a much lower human irritancy than formulas of equivalent sun protection factor.
These and other objects of the present invention will more readily become apparent from the description and examples which follow.